1. Field of the Invention
The present invention relates to a semiconductor switching device. In particular, the invention relates to a semiconductor switching device having a high dv/dt capability.
2. Description of the Prior Art
Heretofore, when semiconductor switching devices are to be implemented in an integrated form, a plurality of lateral thyristors such as shown in FIG. 1, for example, are formed in respective islands isolated from one another by a pn-junction or a dielectric isolation layer with wirings to the electrodes of the lateral thyristors being made so that desired characteristics are obtained. Associated circuits are formed in the same semiconductor substrate. In FIG. 1, numerals 1, 2 and 3 denote an anode electrode, a cathode electrode and a gate electrode, respectively, while numeral 4 denotes a cathode region, 5 denotes a gate region, 6 denotes an anode region, and 7 denotes a semiconductor substrate. The lateral thyristor shown in FIG. 1 and in more general hitherto known thyristors suffer from drawbacks described below. When a positive voltage is applied to the anode with a negative voltage applied to the cathode electrode, the thyristor takes a blocking or non-conducting state so far as the gate electrode is closed. However, when a voltage v.sub.a which increases as a function of time (i.e. a voltage having a positive value of dv.sub.a /dt) is applied to the anode electrode 1, a displacement current is produced and flows from the anode electrode 1 to the cathode electrode 2. This displacement current will then act as a gate trigger current to drive undesirably the thyristor into the conducting state. Such phenomenon provides a serious problem particularly in applications in which the thyristor is employed in a circuit subjected to various noises and/or operated at a high switching frequency, involving a degraded reliability of the thyristor and imposing restriction to a desired high speed operation. With a view to increasing the dv/dt capability, it is known to connect an external resistor between the gate electrode 3 and the cathode electrode 2 or provide means for internally producing a short-circuit between the cathode region 4 and the gate region 5, thereby to prevent the displacement current from flowing into the cathode region as the gate trigger current. However, these measures in turn constitute a shunt current path for the gate current flowing into the cathode region 4, bringing about a new drawback that the intrinsic trigger current is increased. The increased trigger current is of course disadvantageous particularly in a circuit in which a number of the switching devices are employed since the sum of the gate currents will then amount to a significantly large value. In order to increase the dv/dt capability and at the same time reduce the trigger current, it is conceivable to provide additionally an auxiliary circuit including a transistor between the gate and the cathode region, which however means that degradation in the integration density of the switching elements and the correspondingly increased chip area well as accompanying increased manufacturing costs have to be put up with.